Block bottom bag

ABSTRACT

A block bottom bulk material bag, or satchel bottom bag, is formed from multiple plies of paper and an inner ply of plastic that is adhered to the innermost ply of paper. The bag is formed from roll-stock material that is cut into blanks. The knives that cut the blanks are registered relative to one another so that when the pinch bottom or satchel bottom is formed there is no paper surface exposed to the interior of the bag and the entire interior surface of the bag to which bulk material contained in the bag is exposed is covered with plastic.

FIELD OF THE INVENTION

[0001] This invention relates to multiwall bags of the type commonlyreferred to as “block bottom bags,” and more specifically, to such a bagdesigned so that the entire interior surface area of the bag that isexposed to product contained in the bag is covered with a plastic liner.

BACKGROUND

[0002] Block bottom bags, which are also commonly called “satchelbottom” bags are very useful for holding bulk quantities of material andas such are used ubiquitously in numerous industries. While there arenumerous names that are used to describe the bag, they are characterizedas having multiple paper plies and a plastic liner, typicallypolyethylene, and a folded block bottom that is flat when the bag isfilled. The open top end of such bags may be closed in several ways, forexample by folding it over and “pinching” it shut to seal the bag. Whenthe open end of such bags is closed with a pinched seal the bag iscalled a “pinch block bottom bag.”

[0003] Block bottom bags are manufactured from roll stockpaper—typically a standard kraft paper, and roll stock polyethylenesheet material. Although there are several well-known processesaccording to which the bags may be made, briefly described, the bags arefabricated by laminating or gluing a polyethylene layer to an innermostlayer of paper. Several additional layers of paper, some of which havebeen cut and scored by knife blades, are then bonded (as by gluing) tothe inner paper layer. Each layer of paper and the plastic layer are“stepped” or laterally offset relative to the next adjacent layer asthey are laid down to produce a continuous sheet of a flattened materialthat typically has three or more layers of paper, and a layer ofplastic. Rotating knife blades cut perforations in the blank prior toforming the blank into a tube. The blades are registered relative to oneanother to produce a stepped pattern for each of the layers in thelongitudinal direction. The tube is then bottomed, which is the step inthe process where the block bottom is formed and the bottom of theindividual bags are closed and sealed. The bottoming process involvestucking or folding the sides of the bag inwardly and closing and gluingthe bottom flaps over the folded-in sides. The opposite end of theindividual bags are left open, to be closed and sealed later by, forexample, the customer after filling. For ease of reference herein, blockbottom bags are referred to as BB bags.

[0004] After the block bottom is formed, the open end of BB bags may besealed according to several known processes, for example by folding overthe top layers and heat-sealing the plastic liner to the outer ply ofpaper. Other process steps may often be used as well, for example,printing the outer surface of the outer layer of paper with variousinformation such as the identity of the product that will be held in thebag, the manufacturer of the product, and the net weight of the bag.

[0005] BB bags offer numerous advantages. These include that the emptybags are flat and many empty bags may be stacked into a relatively smallspace. This reduces the costs of shipping many empty bags from themanufacturer to the user. The bags are well suited to holding bulk drymaterials such as granulated products. The bags are easily filled, andonce filled are easily sealed. Once sealed, the bags are strong andtypically avoid sifting or leakage, although many conventional BB bagsadvantageously allow for some migration of air into and out of thesealed bag. Moreover, filled bags have a squared bottom and flatnon-gusseted sides that makes the bags particularly suitable forstacking when filled. As such, the filled bags are easily layered instable stacks on pallets. And in addition to the advantages justdescribed, BB bags are quite economical to manufacture and use comparedto other known bulk bags.

[0006] But despite the many advantages of BB bags, there are alsoseveral problems. One complaint is that conventional BB bags have sharpcorners in the bottom area that may be prone to puncture and otherdamage caused by impact. One solution to this problem is offered in U.S.Pat. No. 5,553,943, which discloses a BB bag that includes forming asealed bottom end of the plastic liner that is freely detachable fromthe paper plies. This construction provides a bag that is air-tight whenfilled and sealed, and is said to avoid problems caused by the sharpcorners found in standard BB bags. However, while the bag disclosed inthe '943 patent is useful in many instances, it is not appropriate foruse in all situations where bulk materials need to be packaged. Forexample, in some situations an airtight bag is not desired. Also, theinner plastic liner in a bag such as that described in the '943 patentmay not conform to the shape of the block bottom when the bag is formedand filled. This can result in the bag not standing upright duringprocessing between the filling station and the next operation. Sincemost filling operations are highly automated, such bag handling failurecan be a significant problem. Moreover, the bag of the '943 patent maybe more expensive to manufacture and may require specialized equipmentcompared to conventional BB bags.

[0007] Another, more significant problem inherent in conventional BBbags has to do with paper that remains exposed to the interior of thebags, and thus to the product contained in the bag. Because as noted BBbags are manufactured with the paper laid down in stepped fashion, whenthe bottom of conventional bags is formed, there are necessarily tworelatively small tabs of paper that remain exposed on the interior ofthe bag. Stated in another way, owing to manufacturing constraints inmaking a conventional stepped bag, there are two small tabs of paper onthe interior bottom of the bag that are not covered with thepolyethylene liner, thus exposing paper to the interior of the bag.

[0008] This structural characteristic of conventional BB bags is knownto cause several problems. First, when the bags are used foragricultural commodities, exposure of the commodity to paper is aconcern with regard to contamination. The polyethylene sheet materialthat is used to line BB bags is less prone to contamination, such asbacterial contamination than paper. It is undesirable to have anagricultural product exposed to paper that typically is not sterilized.

[0009] Further, paper can absorb oils and other fluid components thatmight be present in the dry bulk products, while polyethylene typicallywill not absorb oils and other fluids. Absorption of oils and the likefrom the product into the paper can result in damage to the bag anddamage to the product.

[0010] Accordingly, in view of the shortcomings inherent in conventionalBB bags, there is an opportunity to supply a bag that has all of theadvantages of conventional bags, yet avoids the shortcomings.

[0011] The illustrated embodiment of the present invention is just sucha bag. Most significantly, the bag of is constructed in a manner that iteliminates the paper tabs exposed to the interior of the bag that arenotorious in conventional BB bags. Product contained in a bagconstructed according to the present invention is exposed to only thepolyethylene liner and does not touch the paper plies. This providessubstantial advantages from a product quality perspective and mayimprove sanitation of the product contained in the bag. The bag isequally as economical to manufacture as prior conventional BB bags, andmay be manufactured with standard equipment used to make conventionalbags, with appropriate modifications.

[0012] The bag of the present invention is constructed with multipleplies of paper such as kraft paper that are glued together and steppedrelative to one another. The layer of kraft that is adjacent what willbecome the interior of the bag has a layer of plastic laminated onto it.

[0013] The bag is formed from a planar, multi-layer sheet of blankmaterial. Each layer in the blank is stepped in the lateral or cross-bagdirection. The layers are also stepped in some portions of the bag inthe longitudinal direction. The knives that cut the continuousmulti-layer material into individual planar blanks from which theindividual bags are formed have been modified so that the knife registrycreates specific offset or step patterns in the longitudinal direction.As a result of the offset or stepped pattern in the blanks, when theblock bottom is formed in the bag, only plastic liner is exposed to theinterior. A special step formed in the bag prevents leakage or“sifting.”

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The invention will be better understood and its numerous objectsand advantages will be apparent by reference to the following detaileddescription of the invention when taken in conjunction with thefollowing drawings.

[0015]FIGS. 1, 2 and 3 show a conventional BB bag constructed and formedaccording to the prior art.

[0016]FIG. 1 is a perspective view of a prior art BB with the blockbottom formed and the upper portions of the bag removed to expose theinterior bottom of the bag, and illustrating the paper tabs that areexposed in prior art bags.

[0017]FIG. 2 illustrates a prior art BB bag folding in a flat condition,and in which the bottom has been fully formed and the flaps glued inplace.

[0018]FIG. 3 is a plan view of an unformed multi-wall BB blank formedaccording to the prior art, before the blank is formed into a tube.

[0019]FIGS. 4 through 13 illustrate the BB bag of the present invention.

[0020]FIG. 4 is a plan view of an unformed multi-wall BB blank formed inaccordance with the present invention, illustrating the blank before ithas been formed into a tube and before the block bottom has been formed.

[0021]FIG. 5 is a view of a bag according to the present invention,formed from the blank illustrated in FIG. 4, and folded in a flatcondition.

[0022]FIG. 6 is a cross sectional view taken along the line 6-6 of FIG.5, illustrating the layers of the bag in exaggerated thickness andshowing the overlapping layers forming seams.

[0023]FIGS. 7 through 12 are a sequential series of front elevationalviews that show the steps in the formation of the block bottom.

[0024]FIG. 7 shows a formed tube that is open at both ends and is readyfor having the block bottom formed.

[0025]FIG. 8 is a view of the formed tube of FIG. 7, illustrating themain cross-bag score or fold line and two diagonal fold lines formed inthe tube.

[0026]FIG. 9 is the next sequential step and illustrates the plies atone side of the tube folded upwardly toward the top of the bag at themain cross bag fold line, defining the upper and lower flaps of theblock bottom, the bag corners tucked inwardly, and exposing the fourdiagonal fold lines and two secondary fold lines;

[0027]FIG. 10 illustrates the bag of the present invention in thecondition shown in FIG. 9, adding to that view the adhesive applied tovarious portions of the flaps (adhesive shown in stippling).

[0028]FIG. 11 shows the lower flap folded over onto itself at thesecondary fold line and adhered to the bottom of the bag.

[0029]FIG. 12 is the final step in the sequence, illustrating the upperflap folded over onto itself at the secondary fold line and adhered tothe bottom of the bag to complete the formation of the block bottom.

[0030]FIG. 13 is a perspective view of a bag according to the presentinvention with the block bottom formed and the upper portions of the bagremoved to expose the interior bottom of the bag, and illustrating theabsence of exposed paper.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] The BB bag of the present invention is fabricated from multiplelayers of kraft paper, the innermost layer of which (when the bag isformed) is laminated to the adjacent layer of plastic that lines theinterior of the bag. In some of the drawing figures, the edges of thevarious paper plies are shown with line styles that are intended todifferentiate the layers. Thus, with reference for example to FIG. 3,the edges of the outermost or external layer of kraft (“outermost”meaning the outer or external layer in the finished bag) are shown witha solid line. The edges of the middle ply of kraft are illustrated witha regular dashed line. And, the edges of the innermost ply are shownwith a line comprising dashes separated by two dots. In a drawing thatuses this line convention, where the edges of two or more layers arecoextensive, the line is shown as solid. To aid in clarity, referencenumbers in the 100 series are used to identify structural elements andthe like associated with the outermost kraft ply. Reference numbers inthe 200 series are used to identify structural elements in the nextadjacent ply, and numbers in the 300 series are used to identifystructural elements in the third, or innermost kraft layer. Finally,also for the purposes of clarity, like reference numerals are usedthroughout the specification to identify the same element in variousfigures.

[0032] Although the bag of the present invention is disclosed in termsof a preferred embodiment in which the bag has four plies (three pliesof paper and an inner ply of plastic), the principals of the inventionas claimed are applicable to BB bags having both greater and fewerplies. As such, the number of plies described herein is not a limitationof the invention.

PRIOR ART

[0033] A conventional prior art BB bag 10 is shown in FIGS. 1 through 3.With reference to FIG. 1, the outermost layer, also called a ply,(typically kraft paper) 100 is glued to an adjacent middle ply 200,again typically kraft. Middle ply 200 is likewise glued to an inner ply300 (kraft). Inner ply 300 has a sheet of plastic 400 glued thereto.Plastic ply 400 is typically a polyethylene material, but those skilledin the art recognize that there are many materials available suitable asliner material, and the selection of the material used for the linerwill depend upon numerous variables, including for example the nature ofthe material to be held in the bag, the environmental conditions the bagwill be exposed to, and the like.

[0034] During formation of the blank from which the finished bag will beformed, each layer or ply in bag 10 is laterally stepped or offsetrelative to the next adjacent layer, as best illustrated the crosssectional illustration of FIG. 6, so that when a tube is formed from theblank the overlapping edges of each layer form a longitudinal seam thatis laterally offset from the seam in the next adjacent layer. It shouldbe noted that the bag according to the present invention is identical tothe prior art bag with respect to the cross sectional configurationshown in FIG. 6. Therefore, referring specifically to FIG. 6, it may beseen that outer lateral edge 102 of outer layer 100 overlaps with theopposite outer lateral edge 104 of the same layer. In the same way, theouter lateral edge 202 of middle layer 200 overlaps with the oppositeouter lateral edge 204. Outer lateral edge 302 of inner layer 300overlaps with outer lateral edge 304, and the opposite lateral edges 402and 404 of plastic ply 400 overlap and are sealed to one another. Theopposite edges of each layer are overlapped and a strip of suitableadhesive such as hot melt glue is applied to the overlapped area. Asdetailed below, as the blank is formed the plastic ply is glued onto theinner facing surface of inner ply 300 with plural lines of adhesive suchas hot melt applied between these two plies. As such, plastic ply 400 islaminated to inner layer 300 over the entire surface of the plastic plythat contacts layer 300. Plastic ply 400 is laterally offset from innerply 300 in the same manner described above with respect to the otherlayers as shown in FIG. 6.

[0035] Referring to FIG. 3, a single blank 12 formed according to theprior art is illustrated in a planar condition prior to its being formedinto a tube. For reference purposes, the bag axis is defined as the axisextending in the direction of arrow A—that is, the axis that extendsalong the longitudinal axis of the formed bag. The cross-bag axis istransverse to the bag axis, and is illustrated with arrow B. It will beappreciated that continuous roll stock material is used to form acontinuous sheet of multi-layer blank in which the kraft layers areglued to one another and the plastic ply is laminated to the inner layerof kraft as described above. As the roll stock is laid down, each ply isshifted along the longitudinal or bag axis relative to the adjacentlayer or layers, creating a stepped pattern in along the cross-bag axis.The layers of kraft are glued to one another such that they arelaterally offset or stepped relative to the next adjacent ply. As shownin FIG. 3, the opposite lateral edges 102 and 104 of outer ply 100 arelaterally offset from the opposite lateral edges 202 and 204 of theadjacent kraft ply, middle ply 200, and so on. Plastic ply 400 is notshown in FIG. 3 so that the other layers may be shown more clearly.Nonetheless, as noted, it is laterally offset from the inner layer ofpaper 300.

[0036] The layers comprising bag 10 are also longitudinally offsetrelative to the next adjacent layers along the bag axis. Rotating knivesthat cut the various layers form these offsets, however, rather thanphysically shifting or staggering of the layers as they are laid down asdescribed above with respect to the lateral offsets. The knives also cutvarious cut out segments when forming the blanks. The knives cut eachindividual layer separately prior to the layers being glued to oneanother. Again referring to FIG. 3, inner ply 300 is seen as being cutinto a regular rectangle having linear edges, that is, opposite lateraledges 302 and 304 are straight and parallel, and top end edge 306 andbottom end edge 308 are straight and parallel. Although not shown inFIG. 3, plastic ply 400 is laminated to the inner facing surface ofinner layer 300 with the top and bottom edges of the plastic cutcoextensive with top and bottom edges 306 and 308, but with the lateraledges 402 and 404 laterally offset from the lateral edges of adjacentlayer 300. Middle ply 200 is cut so that it is offset from inner ply 300along the bag axis, so that the top edge 206 and bottom edge 208 arestepped from the respective top and bottom edges 306 and 308 of innerlayer 300. Moreover, the top end edge 206 of middle ply 200 includes acut out segment 210 that extends between longitudinally extending foldlines 14 and 16, each of which extends along the length of blank 12along the bag axis. It will be appreciated that because the cut outsegment 210 is cut with a rotating knife as the blank material isconveyed past the knife, forming the cut out segment 210 necessarilyforms a complementary extended section 212 that extends between foldlines 14 and 16 at the opposite or bottom end of the blank. Finally,outer ply 100 is cut such that it is offset along the bag axis frommiddle ply 200. Thus, top end edge 106 is offset from edge 206, andbottom edge 108 is offset from bottom edge 208. As with middle ply 200,outer ply 100 has a cut out segment 110 that extends between fold lines14 and 16, and a complementary extended section 112 that extends acrossthe same distance. In the prior art bag shown in FIG. 3, the top endedges 106, 206 and 306, and the bottom end edges 108, 208 and 308 arelongitudinally offset from one another across the entire width of thebag, including in the portion of the bag between fold lines 14 and 16.

[0037] Fold lines 14 and 16 define the front panel of bag 10, which inFIG. 3 is identified with reference number 18, and the back panel of thebag, labeled with reference number 20. It will be noted that the backpanel is divided into two sections, one on either side of front panel18, and that the stepped pattern in the various plies is the same on allparts of the back panel so that when the blank is folded along foldlines 14 and 16 to form a tube, the back panel sections at oppositesides of the blank are overlapped and sealed.

[0038] Prior to forming blank 12 into a tube, slits 22a are cut throughouter ply 100 on the front panel 18 of the blank. Parallel slits 24 aare cut through middle ply 200 outwardly of slits 22 a on the frontpanel. Slits 24 a are slightly shorter than slits 22, and there is noslit in the inner ply 300 or in plastic ply 400.

[0039] Similarly, slits 22 b are cut through outer ply 100 on back panel20 (which as noted above is divided into two sections, one on eitherside of the front panel), and parallel slits 24 b are cut through middleply 200 on the back panel. Slits 24 b are slightly longer than slits 22b. All of the slits are cut through the adjacent bottom edge of therespective ply, as illustrated.

[0040] The purpose of the slits 22 and 24 is detailed below in respectof the preferred embodiment. It will be noted that the slits in eachpreceding overlying ply are sequentially shorter than the slits in theadjacent ply.

[0041] Blank 12 is formed into a tube and the block bottom is formed (ina manner detailed below) to form an empty bag 10 as shown in FIG. 2. Toform the blank into a blank tube, it will be appreciated that the flator planar blank 12 is folded along fold lines 14 and 16, which form thelateral side edges of bag 12 in FIG. 2 and the outer lateral edges ofthe two portions of back panel 20 (i.e., located on opposite sides offront panel 18 in the flat blank shown in FIG. 3) are overlapped andglued as described above. As a result of the cut out segments 110 and210, the upper edges of the outer, middle and inner plies 106, 206 and306, respectively, define an upwardly extending flap 28 on the back side20 of the bag. Flap 28 may be folded over the upper edges 106, 206 and306 on the front panel 18 of the bag after it is filled to seal theupper open mouth of the bag. The flap is typically heat sealed to closethe bag, although other types of seals are commonly used.

[0042] Referring now to FIG. 1, the bottom portion of a prior art bag 10that has been formed from a blank 12 as shown in FIG. 3 is illustrated.Importantly, owing to the manner in which blank 12 is formed, when theblock bottom of bag 10 is formed and the bag bottom is folded over andsealed, two strips of paper 26 are exposed to the interior of the bag.Said in another way, with the prior art bag shown in FIG. 1, plastic ply400 does not cover the two strips identified with number 26, and assuch, these strips are uncoated kraft paper. For the reasons describedabove, these uncoated paper strips are undesirable.

PREFERRED EMBODIMENT OF THE INVENTION

[0043] The preferred embodiment of the present invention will now bedescribed with reference to FIGS. 4 through 12. A blank 12 similar tothe blank 12 that is described above with respect to FIG. 3, butembodying the present invention, is shown in FIG. 4. Blank 12 is aplanar sheet that comprises three layers or plies of paper and aninnermost layer of plastic. The plastic layer is laminated onto thelayer of paper that will become the innermost layer in the formed bag.The plastic is glued to the innermost layer of paper over substantiallythe entire surface of the plastic layer and the plastic layer istherefore non-detachable from the innermost paper layer. This ispreferably accomplished by laying down plural strips of adhesive alongthe bag axis on the innermost paper ply and adhering the plasticthereto. As noted above, the plastic (which is not shown in FIG. 4) islaterally stepped relative to the next adjacent layer of paper so thatwhen the tubular bag is formed the opposite lateral edges of the plasticare overlapped and sealed to form a seam (FIG. 6). Because the plasticis adhered to the paper over the entire surface of the plastic, theplastic is inseparable from the inner layer of paper so that when theblock bottom is formed, the plastic layer conforms to the shape andfolds of the paper layers.

[0044] Referring to FIG. 4, a single blank 12 formed according to thepresent invention is illustrated prior to its being formed into a tube.The bag axis is the axis extending in the direction of arrow A—that is,the axis that extends along the longitudinal axis of the formed bag. Thecross-bag axis is transverse to the bag axis, and is illustrated witharrow B.

[0045] Roll stock material is used to form a continuous sheet ofmulti-layer blank in which the layers of paper are glued to one anotherand the plastic ply is laminated to the inner layer of kraft asdescribed above. The layers of kraft are glued to one another such thatthey are laterally offset from one another along the cross-bag axis, asshown. The opposite lateral edges 102 and 104 of outer ply 100 arelaterally offset from the opposite lateral edges 202 and 204 of theadjacent kraft ply, middle ply 200, and so on.

[0046] The blank 12 and thus bag 10 according to the present inventiondiffers from the blank and bag formed according to the prior art in themanner that rotating knives that cut the continuous sheets of planarroll stock paper that form the blank are registered with respect to oneanother, and thus the manner in which the blank and block bottom areformed.

[0047] With continuing reference to FIG. 4, longitudinal fold lines 14and 16 extend along the length of the bag along the bag axis and definea front panel 18 and a rear panel 20. The top edge 30 of blank 12 onrear panel 20 is cut with rotating knives so that thee three layers ofpaper (100, 200 and 300) are stepped relative to one another along thebag axis as shown. The identical stepping pattern is formed on rearpanel 20 on the opposite side of fold line 16 at top edge 30. The topedge 32 of front panel 18 (that is, the top edge of the blank betweenfold lines 14 and 16) is cut such that all three layers of paper and theplastic are coextensive—that is, the edges are co-linear, except at acut out step segment 34 that is cut from the outer ply 300. The functionof step segment 34 is detailed below.

[0048] Turning to the bottom edge of the blank 12 of FIG. 4, the threelayers of paper are stepped relative to one anther at bottom edge 36 ofrear panel 20 in a complementary manner to the top edge 30. Bottom edges108, 208 and 308 are thus stepped in the longitudinal direction alongthe bag axis as illustrated. The paper layers at the bottom edge 38 offront panel 18 are, however, cut such that they are coextensive and theedges are co-linear, except for the extended step segment 40 of outerply 100 that is complementary to and a result of step segment 34 cutinto ply 100 at top edge 32.

[0049] As with the blank shown in FIG. 3, in the blank 12 of the presentinvention, ply 300 is seen as being cut into a regular rectangle havinglinear edges, that is, opposite lateral edges 302 and 304 are straightand parallel, and top end edge 306 and bottom end edge 308 are straightand parallel. The plastic ply 400 (which is not shown in FIG. 4) islaminated to the inner facing surface of inner layer 300 with the topand bottom edges of the plastic cut coextensive with top and bottom endedges 306 and 308, but with the lateral edges offset from the adjacentlayer.

[0050] Prior to forming the flattened blank 12 into a tube, slits arecut in the blank in the manner described above with respect to FIG. 3.Specifically, slits 22 a are cut through outer ply 100 on the frontpanel 18, and parallel slits 24 a are cut through middle ply 200outwardly of slits 22 a on the front panel. Slits 24 a are slightlyshorter than slits 22, and there is no slit in the inner ply 300 or inplastic ply 400. Slits 22 b are cut through outer ply 100 on back panel20 and parallel slits 24 b are cut through middle ply 200 on the backpanel. Slits 24 b are slightly longer than slits 22 b.

[0051] The next step in forming a finished bag is forming a tubularblank that has open top and bottom ends from the planar blank shown inFIG. 4. This is done in a standard manner by folding blank 12 along foldlines 14 and 16 such that the opposite lateral edges overlap asillustrated in FIG. 6 and as described above. The overlapping lateraledges are sealed as described above to form the tubular blank.

[0052] The closed block bottom is formed next, and is explained hereinwith reference to the sequence of illustrations beginning with FIG. 7and continuing through FIG. 12. With reference to FIG. 7, the tubularblank is shown in a flat condition with front panel 18 facing the viewerand with fold lines 14 and 16 shown as defining the opposite lateraledges of the flattened tube. When the tube is formed as shown, slits 22and 24 in the respective front and back panels (18 and 20) are alignedwith one another. Diagonal fold lines 50 and 52 are formed by creasingthe tube to facilitate folding. The fold lines extend from therespective lateral edges of the flattened tube and extend to the steppedsegment 34 along bottom edge 38. A primary cross-bag fold line 54 isnext formed laterally across the tube, transverse to the bag axis, andextending between the points where diagonal fold lines 50 and 52intersect the respective lateral edges of the tube (FIG. 8). Fold lines50, 52 and 54 are formed in all plies of the tube. The lower lateralcorners of the flattened tube shown in FIG. 8 are referred to withreference numbers 53 and 55, respectively.

[0053] Turning to FIG. 9, formation of the block bottom of the bag isbegun by folding the plies defining the front panel 18 upwardly alongfold line 54 and by tucking the sides inwardly along diagonal fold lines50 and 52. The corners 53 and 55 are thus folded or tucked inwardlytoward the longitudinal centerline of the bag, as shown in FIG. 9. Thebag thus folded lies essentially flat, and diagonal fold lines 50 and 52define a front flap in front panel 18, denoted generally with referencenumber 56, and a back flap in back panel 20, denoted generally withreference number 58—the two flaps 56 and 58 separated by cross-bag foldline 54. Since the flattened blank is tubular, folding the tube in themanner just described leaves an opening denoted with reference number 59in the bottom of the tube. The opening 59 is defined by the lateraledges 60 of front flap 56, lateral edges 62 of back flap 58, and byupper edge 68 and lower edge 70 of flaps 56 and 58, respectively, andthe perimeter of the opening as just noted defines a regular rectangle.

[0054] Owing to the manner in which bottom edges 36 and 38 of blank 12were formed (detailed above with reference to FIG. 4) and the manner inwhich slits 22 and 24 were cut into the plies (also detailed above), theedges of each layer of material that define lateral edges 60 of frontflap 56 are aligned or coextensive with one another. The end edges ofthe plies of back flap 58 are, in contrast, stepped relative to oneanother as shown so that plies 100, 200 and 300 are stepped relative toone another. Similarly, and again owing to the manner of forming blank12 detailed above, the ply edges at upward edge 68 of front flap 56 arecoextensive except for the stepped segment 40, whereas the ply edges108, 208 and 308 at lower edge 70 of back flap 58 are offset from oneanother.

[0055] Secondary cross-bag fold lines 64 and 66 are formed along thecross-bag axis across front flap 56 and back flap 58, respectively.Secondary fold lines 64 and 66 are parallel to one another and to foldline 54, and equally spaced on opposite sides thereof.

[0056] The next step in the formation of the block bottom is gluing thevarious flaps to one another to close opening 59 in such a manner thatthe interior of the bag thus formed is entirely lined with polyethylenelayer 400. Turning to FIG. 10, adhesive 72 (such as a paste glue or ahot melt glue) is shown by stippled shading. Adhesive 72 is applied tothe exposed surfaces of upper and lower flaps 56 and 58 as shown.Specifically, adhesive 72 is applied to front flap 56 along a zone thatlaterally borders opening 59, and along a complementary zone thatborders opening 59 at back flap 58. However, adhesive is not appliedalong the lower edge 70 of back flap 58. With the adhesive applied asindicated, back flap 58 is folded upwardly along secondary fold line 66and adhered to the now-facing surface, as illustrated in FIG. 11. Thelateral side edges 62 of opening 59 in back flap 58 align with thelateral side edges 60 of front flap 56 in the longitudinal direction(arrow A in FIG. 4) and the lower edge 70 extends just beyond fold line54. Next, front flap 56 is folded downwardly along secondary fold line64 over the already-folded-over back flap 58 and is glued to theouter-facing surface of the back flap. This closes opening 59 and sealsthe opening. Step segment 40 ensures that the seal is complete and leakfree. Because the side edges 60 and 62 of the respective flaps arealigned along the longitudinal axis when the flaps 56 and 58 are foldedover onto themselves as described, the entire inner-facing surface ofthe now-formed bag is lined with polyethylene, and there is no exposedpaper.

[0057] The bag thus formed has an open upper end and is ready forshipment to the end user for filing. The empty bags lie flat and may beefficiently stacked onto pallets and the like for shipping.

[0058] Referring now to FIG. 13, it may be seen that the finished bag ofthe present invention comprises a tubular body having an open end and aclosed end defined by the satchel-fold, also known as satchel bottom,described in detail above. The interior space defined in the bag is freefrom exposed paper—that is, the entire interior surface of the bag towhich bulk material contained in the bag is exposed, is covered withplastic ply 400. This avoids the problems inherent with known BB bagsrelating to exposed paper in the bag interior.

[0059] When a bulk material has been filed into the bag through the openend, the flap 28 (FIG. 5) on back panel 20 is folded over front flap 18and sealed in place to entirely close and seal the bag.

[0060] While the present invention has been described in terms of apreferred embodiment, it will be appreciated by one of ordinary skillthat the spirit and scope of the invention is not limited to thoseembodiments, but extend to the various modifications and equivalents asdefined in the appended claims.

I claim:
 1. In a block bottom bag of the type having a tubular body withan open end and a closed block bottom end to define a bag having aninterior surface, the body comprising multiple plies of paper includingan innermost paper ply and a ply of plastic interiorly of the innermostpaper ply, each of the paper plies having opposite side edges adhered toone another and the plastic ply having opposite side edges sealed to oneanother, and the body further having a front panel and a back panel,opposite side edges and opposed top and bottom edges to define opposedbottom corners, wherein the open end is defined by the respective topedges of the front and back panels and the top edge of the back panelextends beyond the top edge of the front panel, the improvementcomprising: in the front panel the outermost paper ply includes anextended segment extending across a portion of the front panel at thebottom edge, and the block bottom end is formed by folding the opposedbottom corners inwardly at a pair of diagonal fold lines, each of whichextends diagonally from a respective side edge to the bottom edge tothereby define an opening at the bottom of said bag and a front flap anda back flap, wherein the opening is a regular rectangle having opposedside edges and upper and lower edges, the opening having a front flapportion and a back flap portion, wherein the side edges of all paperplies and plastic in the front flap portion of the opening arecoextensive, and the side edges of plastic in the back flap portion ofthe opening are aligned with the side edges of the opening in the frontflap portion but the edges of at least one paper ply in the back flapportion are stepped outwardly relative to the edges of the plastic insaid back flap portion.